Download 21/Interdependence in the Sea

UNIT MARINE
7
ECOLOGY
CHAPTER 21 Interdependence in the Sea
CHAPTER 22 Pollution in the Ocean
CHAPTER 23 Conservation of Resources
There are few, if any, places in the ocean where life
does not exist. The place in which a group of
organisms lives is called a habitat. The inhabitants
of a place such as a coral reef or mangrove swamp
form a community—a group of plants, animals, and
other organisms that interact with one another
and their environment. The study of such relationships among living things and their environment is
called ecology. The study of such relationships in
the ocean environment is called marine ecology.
How do marine organisms interact with one
another to produce a successful and thriving community? What impact do human activities have
on marine life? In this unit, you will study marine
ecology and the role that humans play in marine
communities.
515
21
Interdependence in the Sea
When you have finished this chapter, you should be able to:
IDENTIFY important nutrient cycles in the ocean environment.
EXPLAIN characteristics of oceanic food chains and food webs.
DISCUSS the importance of symbiotic relationships in the sea.
DESCRIBE how succession occurs in marine environments.
21.1
Cycles in
the Sea
21.2
Food Relationships
in the Sea
21.3
Symbiosis in
the Sea
21.4
Succession
in Marine
Environments
516
Some day you will be on your own. But you will not be completely
independent. You will depend on others to help you meet your needs,
and people will depend on you, too. You will need to get a job in
order to provide food, clothing, and shelter for yourself and your family. Your employer will pay you a salary. Medical and dental care also
may be provided. There will be other expenses that you will have to
pay. Life consists of such relationships in which individuals interact
with one another.
In the marine world, there are also many relationships among
organisms. A relationship in which organisms interact in a mutually
dependent way is called interdependence. Some relationships are
based on the need for food. In other relationships, organisms group
together for mutual protection, such as fish swimming in a school.
Yet, not all interdependent relationships are mutually beneficial. In this
chapter, you will study the interactions among marine organisms to
see how these relationships illustrate interdependence.
21.1 CYCLES IN THE SEA
Humans have long used natural resources, although not always
wisely. Some of these resources—such as trees, minerals, topsoil,
fish—have been seriously depleted because they or the products
made from them have been used up or discarded. However, we can
learn an important lesson from nature. There are processes in the
natural world that help conserve resources. One of these processes
involves the recycling of materials. Recycling is a way of conserving natural resources by using them over and over again. Many substances in the marine environment are naturally recycled, too.
Marine Ecology
Imagine that you are on a field trip exploring tide pools along a
rocky coast. In such an environment, you could find snails, mussels, sea stars, and fish in the water. And there would be seaweed
clinging to the rocks. The tide pool is like a miniature ocean, where
you can observe the activities of living things in their habitat. The
study of the interactions of living things with each other and their
environment is called ecology. The study of such interactions
within the ocean is called marine ecology.
The living things in the environment are called biotic factors.
The biotic factors would include all the organisms in the tide pool.
These organisms require the proper environmental conditions to
carry out their life functions. The nonliving factors in the environment are called abiotic factors. Some abiotic factors in the tide pool
are the water, temperature, sunlight, minerals, substrate, and dissolved gases. Together, the interacting biotic and abiotic factors
within an environment make up an ecosystem.
Within an ecosystem, many resources are recycled. Algae use
energy from sunlight, along with carbon dioxide and water, to perform photosynthesis and produce food. You may recall that organisms, such as algae, that make food are called producers. In turn, the
algae are eaten by snails. Animals that eat other organisms are called
consumers. The snail, sea star, mussel, and fish in the tide pool are
consumers. All producers and consumers need certain nutrients that
cycle through the ecosystem. Among the most important nutrients
Interdependence in the Sea
517
that are naturally recycled are carbon, oxygen, nitrogen, sulfur, and
phosphorus.
The Carbon Cycle
By definition, organic compounds contain the element carbon. Photosynthesizing plants and algae take in carbon, in the form of carbon dioxide (an inorganic compound), and produce carbohydrates
(organic compounds). Animals take in carbon compounds when
they eat plants. Animals give off this carbon, again in the form of
carbon dioxide, as a waste product of cellular respiration. This carbon then becomes available to plants for photosynthesis. The movement of carbon through living things in an ecosystem is called the
carbon cycle. (See Figure 21-1.) In the marine environment, the carbon in dissolved carbon dioxide may be taken up by plants and by
animals (such as corals and mollusks) and combined with the calcium in seawater to produce calcium carbonate shells and body
parts. The carbon in these organisms is released into the marine
environment when the organisms die and decay.
Figure 21-1 The carbon cycle.
The Oxygen Cycle
Oxygen is the crucial element needed for cellular respiration in both
plants and animals. Plants (and algae) release oxygen into the
atmosphere when they split water molecules during photosynthesis.
Animals, as well as plants, take in oxygen to use in respiration. Just
518
Marine Ecology
Figure 21-2 The oxygen
cycle.
as plants need the carbon dioxide that animals give off, animals
depend on the release of oxygen by photosynthesizing plants and
algae. This movement of oxygen molecules between plants (algae)
and animals is called the oxygen cycle. (See Figure 21-2.) Together,
the carbon and oxygen cycles are often referred to as the carbon
dioxide–oxygen cycle. Oxygen is also involved in the cycles of other
nutrients in the environment, such as nitrogen, phosphorus, sulfur,
and silicon.
The Nitrogen Cycle
All living things grow. To grow, animals and plants must make
chemical substances called proteins. Important proteins include
hemoglobin, myoglobin, enzymes, and hormones. The main elements that combine to form proteins are carbon (C), hydrogen (H),
oxygen (O), and nitrogen (N). The formation of proteins begins
with algae and plants, which produce the amino acids that build
the proteins. Nitrogen, which is needed to make up the amino
acids, is made available to living things in an ecosystem by a process
called the nitrogen cycle. (See Figure 21-3 on page 520.)
Compounds that contain nitrogen, such as urea and ammonia,
are excreted as metabolic wastes by animals and are used by bacteria.
When plants and animals die, their bodies are decomposed by different kinds of bacteria. Decay bacteria break down the proteins in
wastes and tissues and produce ammonia (NH3), a toxic compound
Interdependence in the Sea
519
Figure 21-3 The nitrogen
cycle.
that cannot be used by plants. Some decay bacteria convert ammonia into the nontoxic ammonium ion (NH4+). Denitrifying bacteria
convert the ammonia into free nitrogen (N2), while nitrifying bacteria oxidize the ammonia to form nitrites (NO2–) and then nitrates
(NO3–). Nitrogen-fixing bacteria, such as blue-green bacteria, also
oxidize the free nitrogen to nitrates (while denitrifying bacteria
reverse that process).
The process of producing nitrates from atmospheric nitrogen is
called nitrogen fixation. Algae and marine plants are able to take
up ammonium ions, nitrites, and nitrates and use them in organic
compounds. The nitrogen in marine plants and algae that are eaten
by animals are used to make proteins. When these plants and animals decay, their proteins are broken down and the nitrogen is recycled. Lightning in the atmosphere also converts free nitrogen in the
air into usable nitrates that fall to the ground in the rain.
The Sulfur Cycle
Another important element that is recycled in the marine environment is sulfur. Many large protein molecules with long amino acid
chains contain sulfur. For example, the protein hemoglobin, which
binds oxygen in red blood cells, has eight sulfur atoms. The sulfur is
520
Marine Ecology
Phytoplankton
and
seaweeds
(Sulfur compounds)
Animal
consumers
Figure 21-4 The
sulfur cycle.
Dead animals
and plants
Absorbed by
green plants
Decay bacteria
Sulfate
SO4
Hydrogen
sulfide (H2S)
Sulfur bacteria
(aerobic)
Sulfur
(S)
Chemosynthetic
bacteria
essential because it acts as a bridge that allows the molecule to bend
and twist, giving it its shape.
Sulfur is recycled from dead matter in the sea back into marine
plants (algae) and animals through a series of chemical reactions
controlled by bacteria. This process is called the sulfur cycle. (See Figure 21-4.) In the first reaction, decay bacteria break down wastes
into hydrogen sulfide (H2S). Hydrogen sulfide (which smells like rotten eggs) is toxic, so plants and algae cannot use it. Chemosynthetic
bacteria convert the hydrogen sulfide into sulfur. But this form is
still not suitable for absorption by algae. Special sulfur bacteria,
however, convert sulfur into sulfate (SO42–), which is absorbed by
the algae and used in the manufacture of proteins. Animals get the
sulfur they need when they eat algae, plants, and other animals.
The Phosphorus Cycle
Living things need energy to do work. Energy is found in molecules
such as glucose. Inside cells, when glucose is oxidized during cellular respiration, energy is liberated and converted into a more usable
form called adenosine triphosphate, or ATP. The energy in ATP is
stored in its phosphate bonds, which contain the elements phosphorus and oxygen. Phosphorus is also present in deoxyribonucleic
acid, or DNA, the genetic blueprint found in all living cells. Thus,
without phosphorus, there is no DNA or ATP, two of the most
important chemicals in living things.
Interdependence in the Sea
521
Figure 21-5 The
phosphorous
cycle.
Phytoplankton
and
seaweeds
Phosphates (PO4)
Animal
consumers
(PO4)
Seabird
droppings
(guano)
Upwellings
Phosphate
nutrients
Dead organic
wastes, inorganic
and organic
phosphates
Phosphorus is supplied to all living things through the phosphorus cycle. (See Figure 21-5.) Decay bacteria decompose dead matter
on the seafloor. The molecules that contain phosphorus, such as
DNA and ATP, are broken down and phosphate (PO43–), a product of
decay, is released into the water. Phosphates from human activities
and from natural sources such as weathered rocks and soil also enter
the marine environment. Plants and algae in the ocean absorb the
phosphates. Marine animals get their supply of phosphorus when
they eat these plants and algae or other marine animals. Another
source of phosphates for marine life is the droppings from seagulls
and other seabirds. Seabirds nest along coasts and on islands, where
thick deposits of their droppings accumulate. These droppings,
called guano, are rich in phosphorus. Since phosphorus is also an
important nutrient for the growth of land plants, guano is often
harvested and sold as fertilizer.
21.1 SECTION REVIEW
1. How are the carbon cycle and the oxygen cycle interrelated?
2. Why are various bacteria so important for the nitrogen cycle?
3. How is phosphorus cycled in the marine environment?
21.2 FOOD RELATIONSHIPS IN THE SEA
Relationships between organisms that are based on nutritional
needs are called food relationships. In the marine environment, just
522
Marine Ecology
as on land, several types of food relationships can be observed. You
will study some of these food relationships next.
Food Chains
Look at the food relationship shown in Figure 21-6. This kind of
relationship is called a food chain, because one living organism
serves as food for another organism, which serves as food for the
next organism in the chain. All food chains begin with a producer.
As you can see in the figure, the producers are the microscopic phytoplankton. The next organism in the food chain is a consumer,
usually an animal, such as the zooplankton. The zooplankton is
called a primary consumer, since it is the first animal in the food
chain. The shrimp, a secondary consumer, feeds on the zooplankton. In turn, the fish or the squid, a tertiary (third-level) consumer,
eats the shrimp. As each organism feeds on another, there is a transfer of chemical energy—from the producer through each level in
the food chain. Each feeding level is called a trophic level.
Food chains vary in length. All food chains must have a producer and at least one consumer. How many consumers are shown
in the food chain in Figure 21-6? As you can see, there are three
consumers. Consumers vary in their food requirements. Animals
such as zooplankton that feed only on algae or plantlike organisms
are called herbivores. The other animals pictured in the food chain
(the shrimp and the herring) are carnivores. An animal that consumes both animals and plants is called an omnivore. Humans are
omnivores, since we eat both plant and animal foods.
When producers and consumers die, their remains are eaten by
scavengers and decayed by decomposers—the organisms that break
down dead matter into smaller particles. The scavengers, such as
Figure 21-6 Feeding levels in a marine food chain;
the first organism is always
a producer.
Phytoplankton
Zooplankton
Shrimp
Herring
Interdependence in the Sea
523
crabs and mud snails, eat the remains of already dead plants and
animals. The most common of the decomposers are the bacteria,
which are found everywhere in the marine environment. However,
most are concentrated in the bottom sediments because the dead
matter on which they feed settles there and accumulates. Some
types of fungi, such as molds, are also decomposers. Another term
used to describe decomposer bacteria and fungi is saprophytes.
The decomposers break down the dead matter into organic molecules and simple compounds, which are taken up by the producers.
This transfer of organic matter—from producers, to consumers, to
decomposers, and back to producers again—“cycles” energy and
nutrients through the food chain. The recycling of organic matter is
the means by which all living things in the ocean can satisfy their
nutritional needs.
Food Webs
In the ocean, many food chains are interconnected, thereby forming a giant food web. Look at the food web shown in Figure 21-7.
The consumers vary in their relationships with one another. The
producers are always at the base of the food web. Most of the consumers shown in the figure are predators; they occupy trophic levels
above the primary consumers. A predator is an animal that kills
and eats another animal. Consumers may also be scavengers, eating the remains of animals that are already dead. In addition, most
of the consumers serve as prey for one or more other animals. The
prey is the animal that is eaten by a predator. Which animals in the
food web are not both prey and predator? All the animals are both
predator and prey, except for copepods and mosquito larvae, which
graze on diatoms, and killer whales, which are the top predators
that are not preyed on by other animals.
What would happen to this food web if the mosquito larvae in
a saltwater marsh were suddenly wiped out by insecticide spraying
along the coasts? It might appear that the killifish would decline in
number (and thus disrupt the food web) because they feed on the
mosquito larvae. However, killifish have another food source, the
copepods, which may sustain them. In food webs, predators often
feed on more than one type of prey. This increases their ability to
survive if one food source becomes scarce. In addition, predators
524
Marine Ecology
Squid
Mackerel
Bluefish
Killifish
Copepod
Snapper
Diatoms
Herring
Larvae
Seal
Figure 21-7 Feeding levels in a marine food web;
most animals are both
predator and prey.
Orca
such as the killifish can be omnivores; that is, they also eat plants.
However, serious disruptions to food chains and food webs—for
example, those that result from El Niño or intense overfishing—
have negative effects on populations of marine organisms such as
seabirds and seals. Similarly, harvesting of krill in Antarctic waters
for human consumption gives cause for concern, since that could
have an impact on baleen whale populations. The whales are secondary consumers that feed directly on these zooplankton.
Food Pyramids
The organisms in food chains can be arranged in a diagram called
a food pyramid. In a food pyramid of numbers, the population at
each higher trophic level is smaller than the one below it. (See
Interdependence in the Sea
525
de
cr
e
as
e
k
Ca
ar
rniv
ore Sh
of
Ca
s
pu
ct
io
n
rn
Di
re
Figure 21-8 Trophic
levels in a marine food
pyramid; the producers
(bottom level) are highest in numbers and in
concentration of food
energy.
ivo
res
Ca
rn
ivo
re
s
to
Oc
r
te
bs
o
L
He
rbi
Pr
ail
vo
re
s
od
uc
er
s
Sn
n
kto
n
pla
yto
Ph
Figure 21-8.) The phytoplankton (algae and plants) outnumber the
snails. There are more snails than lobsters. Lobsters are more
numerous than octopuses. And sharks are the least numerous of
all. In other words, each prey population outnumbers the predator
population above it. What would happen if predators outnumbered their prey? Some predators would die from a lack of food.
Large populations of producers and prey are needed to sustain the
consumers in each higher trophic level. (At times, there may seem
to be more zooplankton than phytoplankton, but the high productivity of the algae supports the continual grazing by the zooplankton.)
The food pyramid also illustrates the transfer of energy from
producers through each successive trophic level of consumers. In
fact, this transfer of energy explains the need for large populations
of producers and prey. Which organism in Figure 21-8 possesses the
highest concentration of available energy? The population of algae
at the base (the largest area) of the pyramid has the highest amount
of available energy. The population of sharks at the top of the pyramid has the least amount of available energy.
Why do the algae have the highest concentration of energy and
sharks the lowest? At each level of consumption, a great deal of
energy is lost as heat. The abundant seaweed is highest in energy
because it absorbs energy directly from the sun and converts it into
glucose. Some of the energy is used for the seaweed’s life functions
526
Marine Ecology
and is lost as heat. When snails consume the seaweed, they use up
most of the energy in the food (about 90 percent), while the rest of
it is stored in the snail’s body tissues.
Only about 10 percent of the food energy consumed by an
organism is available as food to the next organism in the chain.
Thus, when a lobster eats a snail, most of the energy is used for life
processes and is lost as heat, while the remaining 10 percent or so is
stored in the lobster’s body. Consequently, as food energy is transferred from one trophic level to the next, a great amount is lost at
each step. Within the pyramid, there is less energy available for consumption at the trophic level of the shark. To produce 10 kg of body
matter, a shark has to eat about 100 kg of food. But that 100 kg of
food was produced as a result of the consumption of 1000 kg of
food, and so on. That is why the producers and lower-level consumers have to be more abundant than the organisms at higher
trophic levels. So a food pyramid is a way of showing that there is
more energy available at the bottom of a food chain than at the top.
21.2 SECTION REVIEW
1. What are the different trophic levels in a food chain?
2. Explain how an organism can be both predator and prey in a
food web.
3. Why does food energy decrease with each higher trophic level?
21.3 SYMBIOSIS IN THE SEA
In the ocean, there is a constant struggle for existence as marine
organisms avoid predators and compete for food, mates, and territories. Living things have evolved a variety of relationships that
help them survive. One kind of relationship that may be beneficial
to an organism is symbiosis. (See Chapter 7.) In symbiosis, organisms of different species live in close association with one another.
There are three different types of symbiotic relationships, each with
varying degrees of benefit to the parties involved but all related to
feeding strategies.
Interdependence in the Sea
527
Mutualism
Look at the symbiotic relationship shown in Figure 21-9. A clownfish lives among the stinging tentacles of a sea anemone. This special kind of symbiosis is called mutualism, a relationship in which
both species benefit from the association. The sea anemone benefits because it can feed on scraps of food left by the clownfish and is
protected by them from the nibbling of other fish. The clownfish
benefits because it is protected from predators by the stinging cells
in the anemone’s tentacles. The mucus coating on the clownfish’s
skin contains a chemical inhibitor that prevents the discharge of
stinging cells in the tentacles. This kind of mutually beneficial relationship is also found between other small fish and some jellyfish,
and between cleaner shrimps and sea anemones. (See the photograph on page 516.)
Other examples of mutualism can be found in the sea. Many
large tropical fish, such as the grouper, cannot rid themselves of the
parasites that attach to their skin and gills. However, some tiny fish,
such as the cleaning wrasse, feed on fungi and invertebrates that
live on the fish’s skin and in its body cavities. The relationship
between coral polyps and the zooxanthellae (algae) that live inside
them is another example of mutually beneficial symbiosis. Both
Figure 21-9 The clownfish
and sea anemone have a
symbiotic relationship
called mutualism.
528
Marine Ecology
organisms benefit because the algae get a home, carbon dioxide,
nitrate, and phosphate from the polyps, while the coral polyps get
glucose and oxygen from the algae. Mutually beneficial zooxanthellae are also found in the mantle tissue of the giant clam Tridacna
of the South Pacific.
Commensalism
In another symbiotic relationship, called commensalism, one
species benefits while the other species is, apparently, unaffected by
the association. In several species of sharks, a remora (attached by a
dorsal suction disk) or a small group of pilotfish may swim below
or ahead of the shark as it cruises along. (See Figure 21-10.) The
small fish scavenge on leftover bits of food after the sharks have fed.
The shark is neither helped nor harmed by the presence of these
fish; the small fish benefit by getting a free meal.
Another commensal relationship is that of a whale and the barnacles that live on its back. The barnacles benefit by being attached
to a substrate from which they can filter-feed as the whale swims
through nutrient-rich waters. The whale is, for the most part, unaffected by the presence of the barnacles. Organisms such as barnacles, sea anemones, and slipper shells that attach to the bodies of
larger animals without causing them harm are called commensal
organisms.
Figure 21-10 The
shark and the remora
have a symbiotic relationship called commensalism.
Parasitism
A third type of symbiosis, called parasitism, involves a relationship
in which one species benefits while the other species is harmed. An
organism that lives in or on another creature and feeds off its tissues
Interdependence in the Sea
529
is called a parasite. (See Chapter 8.) The animal that is infected is
called a host. A parasite can survive only if it is attached to its host.
If the host dies, so does the parasite, unless it is able to find a new
host.
Because of the loss of commercially valuable fish, parasitism can
be costly to the economy. In the Great Lakes, the lake trout is in
decline due to the accidental introduction of the sea lamprey, which
parasitizes lake trout. (Refer to Figure 12-3, on page 283.) With its
suckerlike mouth and rough tongue, the sea lamprey attaches to a
trout, then scrapes a hole in the trout’s skin and feeds on its body
fluids. The lamprey may feed on the trout for a few hours or several
days, then move on to another trout to continue feeding. The deepsea hagfish takes its parasitism a step farther—it burrows into the
bodies of larger fish and feeds on their tissues from inside.
The life cycle of a parasite often involves more than one type of
host. Black spot disease is a kind of parasitism that occurs in some
marine fish and is caused by a trematode, a type of flatworm. (Refer
to Figure 8-1, on page 194.) Sea gulls that feed on fish with black spot
disease eat the parasitic worms as well. The flatworms reproduce in
the intestines of the sea gull, and the eggs are eliminated with the
bird’s droppings. The eggs develop into small swimming larvae in
the shallow waters of the intertidal zone. The larvae then enter the
first intermediate host, usually a small marine snail, where they
develop into juvenile worms. They leave the snail and attach themselves to the second intermediate host, a passing fish, where they
form black cysts on the skin and then mature. A serious infection
can weaken the fish to the point where it dies. Sea gulls feeding on
these fish consume the mature flatworms, and the cycle continues.
Although parasitism is harmful for the host involved, it is an
important feature of life in animal populations. Most major groups
of marine organisms have internal parasites, usually some type of
flatworm. Many ocean fish (such as the grouper mentioned above)
also have external parasites. These are usually tiny crustaceans, such
as isopods and copepods, that attach to the host’s skin and gills and
obtain their nutrients from the fish’s blood. Environmental conditions can also play a role in determining an animal’s susceptibility
to parasitism. Scientists have noticed that poor water quality
increases stress in marine organisms and lowers their resistance to
disease, making them more vulnerable to parasitic infections.
530
Marine Ecology
21.3 SECTION REVIEW
1. How do symbiotic relationships illustrate interdependence?
2. How does the clownfish survive among the stinging tentacles
of the sea anemone? Why is that an example of mutualism?
3. Explain the kind of symbiosis that sharks and pilotfish have.
21.4 SUCCESSION IN MARINE
ENVIRONMENTS
What was it like growing up on the street where you live? Do you
remember new neighbors arriving and old friends moving away?
New buildings may have been constructed on your block or others
remodeled. Over time, neighbors and neighborhoods change.
Change also occurs in the marine environment. You can place a
hard object, such as a rock, in a tide pool at the beach and examine
it over a period of time. At first, a thin coat of algae will begin to
cover the rock’s surface, making it feel slimy. If you leave the rock in
a tide pool long enough, you may see its coat of algae replaced by
barnacles. Later on, seaweed, mussels, and other small organisms
may join the barnacles that coat the rock. The same process also
occurs on the hull of a boat. In fact, boats sometimes need to have
their hulls scraped clean of barnacles and mussels, because they
affect the vessel’s movement through the water.
Ecological Succession
The process by which one community of organisms gradually
replaces another community of living things over time is called ecological succession, or biological succession. Succession occurs in a
variety of marine habitats, such as sandy beach dunes, rocky shores,
and coral reefs. (See Figure 21-11 on page 532.)
What causes one community of organisms to replace another
community? When the chemical and physical conditions in an area
are no longer suitable for the existing life-forms, a new community
Interdependence in the Sea
531
Figure 21-11 Succession
on a sandy dune: beach
grasses are followed by
shrubs.
of living things slowly takes over. The first group of organisms to
appear in an area devoid of life—such as on a new wharf piling or
new volcanic island—is called the pioneer community. The pioneer organisms represent the first stage in ecological succession.
When the pioneering algae settle down on a rock, they begin to
change their physical and chemical environment. Algae take up carbon dioxide, make their own food, and produce oxygen. Changes
brought about by the algae actually favor colonization by other
organisms, such as barnacles. The barnacles, in turn, alter the environment and are succeeded by a more stable community, dominated by mussels. The last stable community to appear in any
succession is called the climax community. Each climax community has a dominant species, which is the main species that finally
appears there. Marine succession varies with climate. The algae-tobarnacle-to-mussel succession occurs along temperate shores in the
more northern latitudes.
In tropical seas, corals form the climax community. Coral
polyps attach to any hard substrate and, over a long time, cover the
substrate with a coral reef. Other organisms are attracted to the
corals, slowly forming a more complex and diverse community. In
fact, old sunken ships are difficult to find in tropical waters because
this process of succession entombs ships under coral.
532
Marine Ecology
ENVIRONMENT
Turf Wars off Fisherman’s Wharf
San Francisco is a multicultural city where people
from all over the world have settled. Paralleling
this influx of people from foreign lands has been
the arrival of foreign, or exotic, marine species
from other oceans to the waters of San Francisco
Bay. But not all the inhabitants get along. Turf
wars have developed between some of the
native species and the exotic invaders. So far,
marine biologists have identified more than 200
exotic species in the bay, and new ones continue
to arrive. These organisms are usually introduced
in the ballast water that is discharged daily by
ships anchored in the bay. Water from the ballast
tanks contains eggs and larvae of fish and invertebrates from the ports where the ships
embarked. Some species arrive packed in boxes
of fish bait. Crabs from China, protozoans from
Japan, and a variety of species from the East
Coast—including Atlantic ribbed mussels (see
photograph), striped bass, and cordgrass—have
found a niche in the shallow bays and inlets of
the San Francisco estuary.
This great increase in species diversity has
come at a price. Competition with, and predation by, the exotic fauna and flora has decreased
populations of some local species. The thicktail
chub, a native to the bay, has been eliminated
by the influx of foreign species. Another local
fish, the delta smelt, is now an endangered
species. The Atlantic green crab—originally
a native of Europe that “emigrated” to New
England coastal waters in the 1950s and later
traveled in ballast tanks to San Francisco Bay—
is a predator that attacks the juvenile Dungeness
crab, a popular food item in the bay area. One
exotic, a burrowing isopod from Australia, presents a different kind of problem: it tunnels into
the Styrofoam blocks that support floating
docks, causing them to sink. On the other hand,
some of the exotic species have turned out to
be valuable new food sources for the bay area.
The ultimate effects of the exotics in the
bay are yet to be seen. Marine scientists think
that, over time, a new ecosystem will establish
itself—with a climax community of many different marine species replacing the ecological roles
of former inhabitants that lost the fight for their
turf off Fisherman’s Wharf.
QUESTIONS
1. Identify two exotic species in San Francisco Bay. Why are they called exotic?
2. How did the various exotic marine species arrive in San Francisco Bay?
3. Why are some exotic species harmful to the native bay species? Give an example.
Interdependence in the Sea
533
Climax communities are not necessarily permanent. Natural disturbances such as storms can strip mussels from rocks. The activities of people can have an effect, too. For example, overharvesting
of mussels from rocks can wipe out a climax community. When a
climax community is destroyed, the area is made available for pioneering organisms to start the process of succession over again. It
takes time, perhaps many years or decades, for a new climax community to become established.
Succession on Islands
Figure 21-12 Succession
on an island, from pioneer
to climax plant species.
Lichens
534
Mosses
Marine Ecology
Succession occurs on land as well as in the sea. In areas of seismic
activity, such as the South Pacific, tiny islands are born each year as
molten lava from the seafloor reaches the surface and solidifies into
rock. At first, these islands are devoid of life. But after the rock cools,
conditions for life become more favorable. Spores and seeds arrive
on the new islands, brought by winds, waves, and various animals.
For example, seeds may be carried in the digestive tracts of migrating birds. The first pioneer organism to appear on barren rocks is
usually lichen. The lichen is actually composed of two organisms,
an alga and a fungus, growing together in a symbiotic relationship.
Acids in the lichens erode the surface of rocks, producing a more
porous surface and loose sediments that favor the growth of the
next community, the mosses. Mosses carpet the surface of rocks and
cause further erosion into rock particles and gravel. The loose sediments and dead plant matter create a thin soil that is favorable for
the germination of grass seeds. Grasses take over and dominate the
landscape for a number of years and then are succeeded by shrubs.
The shrubs cause further changes in the soil that favor the growth of
trees, the climax community (of plants) that will dominate the
island. (See Figure 21-12.)
Grasses
Shrubs
Trees
21.4 SECTION REVIEW
1. What causes one community of organisms to succeed another?
2. Compare one pioneer community with one climax community.
3. How does succession occur on a rocky volcanic island?
Interdependence in the Sea
535
Laboratory Investigation 21
Observing How a Barnacle Filter-Feeds
PROBLEM: How does the barnacle obtain its food?
SKILLS: Conducting an experiment; observing the behavior of organisms.
MATERIALS: Live barnacles attached to substrates, bowls or shallow containers, seawater, hand lens, watch or clock, fish food (dried plankton).
PROCEDURE
1. Put a barnacle with its attached substrate in an empty bowl or container.
Notice the overlapping shells that surround and protect the barnacle. Make
a sketch of the barnacle in your notebook. Observe the barnacle closely and
note if it moves or responds to the environment in any way.
2. Cover the barnacle with seawater. Wait a minute for the barnacle to respond.
Notice the appendages, called cirri, as they extend outward and then retract
into the barnacle’s shells. This movement is an automatic response by the
barnacle to its seawater environment.
3. Count the cirri and record the number in your notebook. Notice the hairlike
bristles attached to the cirri. Use your hand lens for a close-up look. Food
particles get trapped in the bristles as the cirri sweep the water. The movement of the cirri in and out of the shells is the barnacle’s method of filterfeeding. Make a sketch of a filter-feeding barnacle; label all parts.
4. You can measure the automatic feeding response of the barnacle by calculating the number of times the cirri beat per minute. Count the number of
times the cirri move in 15 seconds and multiply by 4 to get the number of
movements per minute. Record this number in the Automatic Response column in your copy of Table 21-1. Try to perform six trials to obtain an average
number of cirri movements per minute. (This is the control group.)
536
Marine Ecology
TABLE 21-1 COMPARING FEEDING RESPONSES OF THE BARNACLE
(MOVEMENTS/MINUTE)
Trials
Automatic Response
Response to Food
1
2
3
4
5
6
Total
Average
5. Now sprinkle some fish food in the water, near the barnacle. Again, count
the movements for 15 seconds and then calculate the number per minute.
Record the number in the Response to Food column in your copy of Table
21-1. Try to perform six trials to obtain an average number of cirri movements per minute. (This is the experimental group.) Compare the results
from both sets of trials to see if there are any significant (measurable) differences.
OBSERVATIONS AND ANALYSES
1. Does the barnacle show any feeding response when it is not covered with
seawater (that is, when it is exposed to the air)?
2. Compare the feeding responses of the barnacle in the presence and in the
absence of food. Is there a measurable difference?
3. Explain how the barnacle is adapted for filter-feeding. What special structures and functions did you observe that help the barnacle survive?
Interdependence in the Sea
537
Chapter 21 Review
Answer the following questions on a separate sheet of paper.
Vocabulary
The following list contains all the boldface terms in this chapter.
abiotic factors, biotic factors, climax community, commensalism,
ecological succession, ecology, ecosystem, food chain, food
pyramid, food web, lichen, marine ecology, mutualism, nitrogen
fixation, omnivore, parasitism, pioneer community, predator, prey,
primary consumer, scavengers, secondary consumer, tertiary
consumer, trophic level
Fill In
Use one of the vocabulary terms listed above to complete each sentence.
1. A relationship in which both species benefit is called
____________________.
2. A ____________________ shows the trophic levels in a food chain.
3. The ____________________ is the first group of living things in an area.
4. Zooplankton would be the ____________________ in an ocean food chain.
5. The nonliving factors in an environment are called ____________________.
Think and Write
Use the information in this chapter to respond to these items.
6. What is the difference between mutualism and commensalism?
7. Why are producers more abundant than consumers in a
habitat?
8. Explain why a climax community is not always permanent.
Inquiry
Base your answers to questions 9 through 11 on Figure 21-7 on page
525, which shows a marine food web, and on your knowledge of
marine science.
538
Marine Ecology
9. In terms of numbers, which is the most abundant organism in
the food web? Explain.
10. Which organism in the food web is the least numerous?
Explain why.
11. How would a sudden decline in the herring population affect
the food web?
Multiple Choice
Choose the response that best completes the sentence or answers the
question.
12. Which statement is most correct
about the relationship between
the two fish shown here?
a. Both fish benefit equally.
b. The shark benefits and the
remora is harmed. c. The remora benefits and the shark is
harmed. d. The remora benefits and the shark is unaffected.
13. The ecological cycle
shown here provides
plants in the marine
ecosystem with
a. carbon to carry out
photosynthesis
b. phosphorus for ATP
production c. oxygen
to carry on aerobic
respiration d. a source
of nitrogen for protein
synthesis.
14. A producer–consumer interaction in the marine ecosystem is
illustrated by a. bacteria feeding on detritus b. sea gulls
scavenging on dead clams c. snails grazing on algae
d. a fish eating a shrimp.
15. The nitrogen cycle makes it possible for marine algae to
a. carry on photosynthesis b. breathe c. produce ATP
d. synthesize proteins.
Interdependence in the Sea
539
16. The flow of energy in a food chain occurs as follows:
a. consumer to producer to producer b. decomposer to
producer to consumer to producer c. producer to consumer
to decomposer to consumer d. producer to consumer to
decomposer to producer.
17. The carbon cycle and the oxygen cycle directly involve
a. consumers and decomposers b. producers and
consumers c. predators and prey d. predators and
consumers.
Base your answers to questions 18 through 21 on the following food
relationship: algae → shrimp → lobster → octopus.
18. This sequence illustrates a a. food web
c. food chain d. food cycle.
b. food pyramid
19. The population of which organism has the highest food
energy? a. algae b. shrimp c. lobster d. octopus
20. Which organism is the most numerous in its habitat?
a. algae b. shrimp c. lobster d. octopus
21. Which organism is both predator and prey here?
b. shrimp c. lobster d. octopus
a. algae
22. The cleaning wrasse that eats parasites living on a grouper’s
skin provides an example of a. commensalism
b. succession c. mutualism d. parasitism.
23. The correct sequence for plant communities on an island is
a. mosses, lichens, shrubs, trees, grasses b. lichens, mosses,
grasses, shrubs, trees c. grasses, mosses, lichens, trees,
shrubs d. lichens, grasses, mosses, trees, shrubs.
Research/Activity
With a team of classmates, set up a saltwater aquarium tank containing some local marine species, such as killifish, snails, seaweed,
shore shrimp, and mussels. Identify the producers, consumers, and
decomposers (scavengers) in the aquarium. Prepare a poster board
that shows the food relationships among the different organisms.
(Remember: Do not overstock the tank, and be sure to use proper
filtration and aeration.)
540
Marine Ecology